A problem prevalent in the communication of electronic data, especially, image or video data, is offset drift and gain drift. Offset and gain drift can be caused by the characteristics of individual components responsible for processing or communicating the image data; i.e., a shift register functioning as a buffer for the signals received from a charged coupled device (CCD) may have inherent offset and gain characteristics unique to itself or a scanner may contribute to offset and gain drift due to the present operating conditions; i.e., the operating temperature, light temperature, age, etc. Moreover, offset and gain drift may be attributed to the individual characteristics of the channel transporting the data from one component to another during the processing cycle. If offset drift or gain drift is not adequately addressed; i.e., the signal being processed is not adjusted to counteract the offset or gain drift; the processing of the signal will not be accurate which, in an image processing system, can cause the generated picture or image to have a lower quality.
In systems employing image viewing devices, such as charge coupled devices (CCDs), for viewing by raster scanning an original, the output signal produced by the CCD includes a potential attributable to the inherent operating characteristics of the CCD. To restore the image output signal of the CCD to a true or absolute value, the potential derived from the CCD, referred to as the offset potential or signal, must be removed from the image signal. However, if the offset signal that is removed is greater or less than the actual offset signal, a noticeable aberration or distortion in the image output signal may result. Since the operating characteristics of a CCD often vary widely from one CCD to another and even vary from time to time for the same CCD or for different integration rates, the accurate determination of the offset signal to be removed is often difficult. The problem is further complicated in systems where multiple CCDs are employed.
Operating systems utilizing the afore-mentioned image viewing devices are designed for a fixed image signal gain. However, since the operating characteristics of an individual CCD in an imaging device may vary, the signal gain may vary from one CCD to another, or may even vary for the same CCD. Thus, where a system is optimized for a specific CCD operating at a specific speed, one would have to redesign or recalibrate the system to accommodate a change in gain due to changes in the operating characteristics of the CCD or if the CCD is replaced with another one.
To address these problems, typical image processing systems or image scanning systems perform calibrations of the image sensor once every predetermined number of scans. In most cases, the predetermined number of scans is less than ten, but, many systems calibrate the image sensor prior to each scan. Even though these systems have addressed the problem of offset and gain drift, the compensation techniques used in these systems, as will be discussed below, do not fully compensate for integral changes in offset or gain characteristics and are not readily adaptable to systems which must process data at a high speed, for example, a constant velocity transport image processing apparatus.
An example of a device which performs calibration once every predetermined number of scans is the device disclosed in U.S. Pat. No. 3,952,144 to Kolker. Kolker discloses that a facsimile transmitter makes a preliminary calibrating scan in which the transmitter sequentially scans a known black area and a known white area. An automatic background and contrast control unit stores a first sample of the uncorrected video signal which represents the scanned black area and stores a second sample of the uncorrected video signal which represents the scanned white area. During subsequent scanning, the automatic background and contrast control unit continually produces voltages representing the stored black and white samples and uses these voltages to correct the video signal received during the scanning of the document.
Another example of a device which corrects for offset and gain drift is disclosed in U.S. Pat. No. 4,555,732 to Tuhro. This U.S. Patent discloses an image sensor correction system which maintains the offset voltages in the shift registers of a multi-channel image sensor substantially equal. U.S. Pat. No. 4,555,732 discloses that a pair of control gates permits sampling the current offset voltages in the shift register of each channel to provide an adjusted potential for balancing any differences between the shift registers. More specifically, U.S. Pat. No. 4,555,732 discloses a device which compares the various offsets of a plurality of shift registers and determines a single offset potential to be applied to each shift register according to the comparison.
A device which proposes to correct gain and offset drift due to changes in the operating characteristics of a CCD is disclosed in U.S. Pat. No. 4,216,503 to Wiggins. U.S. Pat. No. 4,216,503 discloses a system where dark and light level signals are isolated and processed by a microprocessor unit in accordance with a pre-established routine to provide an offset potential and gain multiplicand. The determined offset potential and gain multiplicand are used to remove the offset and set a signal gain for the next succeeding line of image signals. The process is then repeated for each line of image signals to be outputted from the CCD.
Although U.S. Pat. No. 4,216,503 discloses a device to correct offset and gain drift on a continual basis, such a process is not adaptable to correct offset or gain drift in a high speed copier or drift in a fast scan direction because this method only corrects for offset drift or gain drift in a slow scan direction. In other words, the technique disclosed by U.S. Pat. No. 4,216,503 adjusts the offset gain value only upon the completion of a scanning of a full line of data.
Another problem associated with the correction of offset and gain drift is the establishment of reference values through calibration. In a typical platen scan configuration, calibration is not a substantial problem since the carriage can scan the calibration target before the scanning of each individual document. However, in a constant velocity transport system, the carriage is stationary, and thus, it is practically impossible to scan a calibration target before each individual scanning of a document. Therefore, with respect to a constant velocity transport system, it is necessary to have a calibration system wherein an unlimited number of scans can be made between actual generation of calibration values and still adequately compensate for offset and gain drift due to the prevailing operating conditions.
To realize this goal, the factors that cause the system to have to be recalibrated have to be corrected. These factors are typically profile drifts due to thermal changes in the sensor bar, video circuits, or the illumination system. The drifts can be in the form of offset changes or gain changes and can occur in the fast scan direction or the slow scan direction. It is noted that there are many methods which address the slow scan drift correction. Among these methods are the lamp intensity control method, automatic gain control method, a D.C. restore method, and the methods discussed above.
However, these various methods have not been able to correct changes in the form of offset and gain that occur in the fast scan direction, nor are these methods effective in a constant velocity transport system. Moreover, with the recent development of full width array systems, the drift changes in the fast scan direction as well as the gain changes in the fast scan direction become more prevalent, notwithstanding the system being used; i.e., platen scan or constant velocity transport. This is due to the fact that the full width arrays are made of several smaller arrays joined together in a butted or staggered manner.
With respect to the problems of drift in a full width array system, the two types of drift that need to be address are offset and gain. Fast scan offset drift is caused by temperature changes and differences between the individual sensor chips or video channels. On the other hand, fast scan gain changes are caused by either changes in the profile of the lamp changing due to thermal operating characteristics of the lamp or by gain drift in the actual sensor chip or the video channels.
Another component or aspect of an image processing system which experiences problems with gain drift and offset drift is the actual channels utilized to transfer or communicate the image data between points within the image processing system. More specifically, in analog video systems, where there are multiple channels of image or video data, it is important that each channel has the same transfer function or response characteristics. Any differences between the channels can produce differences in the final image that is outputted. These differences may show up as channel banding or streaking. Even though each channel might be identical in design, there are various tolerances associated with the components of each channel and hence there will always be a slight difference in the performance for each channel.
The difficulty with the prior art compensation systems, an example is illustrated in FIG. 1, is that these systems cannot compensate for offset and gain drift in the fast scan direction or be readily implemented in a high speed copier configuration as illustrated in FIG. 2. A compensation system must be able to quickly adjust the offset and gain settings for changes in operating characteristics, and more specifically, to characteristic changes realized along a fast scan direction. To achieve this, a system must be able to respond quickly and without generation of calibration values during the scanning process.